The quantitative determination of analytes in body fluids is of great importance in the diagnoses and maintenance of certain physiological physical conditions. For example, lactate, cholesterol, and bilirubin should be monitored in certain individuals. In particular, it is important that individuals who have diabetes frequently check the glucose level in their body fluids to regulate the glucose intake in their diets. The results of such tests may be used to determine what, if any, insulin or other medication should be administered. In one type of blood-glucose testing system, test sensors are used to test a sample of blood.
A test sensor contains biosensing or reagent material that reacts with, for example, blood glucose. The testing end of the sensor is adapted to be placed into the fluid being tested (e.g., blood that has accumulated on a person's finger after the finger has been pricked). The fluid may be drawn into a capillary channel that extends in the sensor from the testing end to the reagent material by capillary action so that a sufficient amount of fluid to be tested is drawn into the sensor. The tests are typically performed using optical or electrochemical testing methods.
Electrochemical test sensors are based on enzyme-catalyzed chemical reactions involving the analyte of interest. In the case of glucose monitoring, the relevant chemical reaction is the oxidation of glucose to gluconolactone or its corresponding acid. This oxidation is catalyzed by a variety of enzymes; some of which may use coenzymes such as nicotinamide adenine dinucleotide (phosphate) (NAD(P)), while others may use coenzymes such as flavin adenine dinucleotide (FAD) or pyrroloquinolinequinone (PQQ).
In test sensor applications, the redox equivalents generated in the course of the oxidation of glucose are transported to the surface of an electrode, whereby an electrical signal is generated. The magnitude of the electrical signal is then correlated with glucose concentration. The transfer of redox equivalents from the site of chemical reaction in the enzyme to the surface of the electrode is accomplished using electron transfer mediators.
Electron transfer mediators previously used with FAD-glucose dehydrogenase (FAD-GDH) include potassium ferricyanide, phenazine-methosulfate (PMS), methoxy phenazine-methosulfate, phenazine methyl sulfate, and dichloroindophenol (DCIP). These compounds, however, have proven to be highly susceptible to the environmental conditions including temperature and moisture, which result in test sensor reagents of low stability. For example, during storage, reduced mediator may be produced from interactions between the oxidized mediator and the enzyme system. The larger the amount of mediator or enzyme, the larger the amount of reduced mediator that is produced. The background current, which increases over time, will generally increase toward the end of the shelf-life of the sensor strips because of the high concentration of reduced mediator. The increased background current may decrease the precision and accuracy of the measurements of the test sensor and, thus, provide a limited shelf-life for the test sensors.
Another disadvantage associated with existing test sensors is the relatively slow fill rate. Achieving a fast sensor fill rate is desirable so that the re-hydration of the reagent may be faster and more uniform. Thus, faster fill rates generally result in more precise, stable test sensors having less variation.
Therefore, it would be desirable to have a reagent that addresses one or more of these disadvantages.